My impression is that it was more common before digital backs became established among the medium-format crew, and before full-frame DSLRs became nearly cheap; but digitization of images at a quality level that was either impossible or unaffordable with digital cameras is actually a fairly old custom:
In the field, where size, weight, and exposure time are at a premium, the generally superior sensitivity and resolution of mature photochemical processes make gathering those photons you wanted doable. Back at the office, some preposterously expensive and dubiously-man-portable scanner(that only has to digitize one slice of the image at a time, not the entire scene) digitizes the results for you. Some of the nuttier ones still use photomultiplier tubes, rather than silicon sensors, which isn’t exactly an option in-camera.
(Of course, while this means that claims of digital images that could only have been shot on chemical film are not necessarily false, the usual quality of the monitors people view them on tends to make them irrelevant-in-context: you can buy very, very, nice monitors for photo work; but people tend not to, so it becomes rather like opining about whether vinyl sounds ‘warmer’ when you are listening on dollar store earbuds…)
